The first autosave point after crashing the tank through the Citadel wall is the start point for this. When you open the door, you'll have to kill three soldiers (two on the ground and one up on a parapet). After you kill them, look near the fence to the left of the door you just came through. On the ground is a square plate. Destroy it with an explosive (hand grenade, time bomb, grenade shot, rocket) and climb down the hole. Turn on your nightvision and head down the tunnel. Blast a hole in the wall around the second corner. Destroy the boards on the floor and then drop down the hole. Duck down and squat-walk to the next ledge. Jump up and head down the hexagonal tunnel that turns right. Be careful. There are three soldiers on the platform above you. One well-placed grenade will kill all of them at once. Jump over the pipe and climb the ladder. On the opposite wall are two launchers and several EMP grenades. At this point, you can proceed out the double doors into the central courtyard, or retrace your path and enter the courtyard the normal way.
Near the end of the Public Information Building, you'll enter the last chamber just after Sopot's escape craft lands on the helipad. Instead of going up the stairs (after killing the two guards), blast a hole in the wall where the stairwell is lined up. Go through the hole and climb the ladder. Up ahead will be the W.A.S.P. And a full pack of reloads. Just before going back down, watch out for the guard. When you attack Sopot's ship, use the W.A.S.P. You'll need 7 direct hits, but, if you stand close to the wall and keep the door open, you can use the guided missile mode and fire your weapon without overly exposing yourself to return fire.
When you enter the sewer and have the cutscene with Echo, when the shooting starts, equip either the Nano GL or NICW and run towards the tunnel straight in front of you where the 'beam' comes from. Just before you reach the opening start firing grenades. Do NOT use the timed grenades (Nano GL). If you time it just right, The enemy will have dropped a Rail Gun and an HMG.
I found this one by accident during a playthrough.